1. Auditory Sensation (Hearing) L13
Faisal I. Mohammed, MD, PhD
University of Jordan

2. Objectives Define decibel (intensity) and Hz (frequency)
Describe the ossicular system and explain its function
Follow up sound transmission up to the cochlea
Outline the structure of cochlea, and the organ of Corti
Describe the mechanism of sound transduction
Follow up the auditory pathway to the cerebral cortex
Describe auditory abnormalities (types of deafness)
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4. Frequency of sound wave: Audible sound wave pure tones 20 – 20,000 Hz
Speed of sound is 335 m/sec in Air
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7. Decibel: a measure of sound intensity
Decibel (dB) = 10 log I/IR
I = intensity of sound, IR= reference intensity
Acoustic intensity is proportional to the square of sound pressure level
Sound pressure is more conveniently measured than sound intensity
Sound pressure level (SPL) unit is decibel
SPL (dB) = 20 log P/PR
P= the sound pressure in N/m2 (N=Newton, m = meter)
PR= reference pressure (either dynes/cm2, the absolute threshold for human hearing and equal 20 micropascal, or 1 dyne/ cm2 )
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8. The Tympanic Membrane and the Ossicular System
Tympanic membrane functions to transmit vibrations in the air to the cochlea
Amplifies the signal because the area of the tympanic membrane is 17 times larger than the oval window (55 sq. mm Vs. 3.2 sq. mm)
Tympanic membrane connected to the ossicles
malleus
incus
Stapes
Ossicular system works as a lever system and amplifies the sound 1.3 time
Total amplification is 22 times (17x1.3) called Impedance Matching (match the resistance of sound wave movement in fluid vs. the resistance in air)
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10. Components of the Auditory System
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13. Attenuation of Sound by Muscle Contraction
Two muscles attach to the ossicles
Stapedius (supplied by facial Nn VII)
tensor tympani (supplied by Trigeminal Nn V)
A loud noise initiates reflex contraction after milliseconds
Attenuates vibration going to cochlea (Attenuation reflex)
Serves to protect cochlea and damps low frequency sounds i.e., your own voice
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14. Cochlea
system of three coiled tubes separated by membranes into the scala tympani, scala media, scala vestibuli
sound waves cause back and forth movement of the tympanic membrane which moves the stapes back and forth
this causes displacement of fluid in the cochlea and induces vibration in the basilar membrane
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15. Structure of the Human Cochlea
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18. BasilarMembrane
contains about 30,000 fibers which project from the bony center of the cochlea, the modiolus
fibers are stiff reed-like structures fixed to the modiolus and embedded in the loose basilar membrane
because they are stiff and free at one end they can vibrate like a musical reed
the length of the fibers increase and the diameter of the fibers decrease from base to the helicotrema, overall stiffness decreases 100 times, high frequency resonance occurs near base, low near apex
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19. Structural Components of the Cochlea
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20. Organ of Corti receptor organ that generates nerve impulses
lies on the surface of the basilar membrane, contains rows of cells with stereocilia called hair cells
the tectorial membrane lies above the stereocilia of the hair cells
movement of the basilar membrane causes the stereocilia of the hair cells to shear back and forth against the tectorial membrane
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23. The Organ of Corti
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24. movement of the basilar membrane causes the stereocilia of the hair cells to shear back and forth against the tectorial membrane.
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25. Scala vestibuli Cochlear duct (contains endolymph) tympani Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2 3 4 5 6
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2 3 4 5 6 7
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2 3 4 5 6 7 8
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2 3 4 5 6 7 8 9
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2 3 4 5
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2 3 4
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2
Scala
vestibuli
Cochlear duct
(contains endolymph)
tympani
Perilymph
Basilar
membrane
Cochlea
Sound waves
Helicotrema
Stapes vibrating
in oval window
Malleus
Incus
External auditory
canal
Tympanic
Secondary tympanic
membrane vibrating
in round window
Auditory tube
Vestibular membrane
Middle ear
Tectorial membrane
Spiral organ
(organ of Corti) 1 2 3
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26. Nerve Impulse Origination
The stereocilia, when bent in one direction cause the hair cells to depolarize, and when bent in the opposite direction hyperpolarize.
this is what begins the neural transduction of the auditory signal
Auditory signals are transmitted by the inner hair cells.
3-4 times as many outer hair cells than inner hair cells
outer hair cells may control the sensitivity of the inner hair cells for different sound pitches
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29. Determination of Sound Frequency and Amplitude (sound intensity)
Place principle determines the frequency of sound perceived.
Different frequencies of sound will cause the basilar membrane to oscillate at different positions (basilar membrane is tonotopically organized)
Position along the basilar membrane where hair cells are being stimulated determines the pitch of the sound being perceived.
Phase-locked (volley) principle at lower frequencies of sound where firing rate determines the phase of sound wave (i.e frequency of sound wave)
Amplitude is determined by how much the basilar membrane is displaced ( by frequency of impulses from the nerve fiber and the No. of nerve fibers stimulated)
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30. The “Place Principle”
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32. Threshold of hair cells
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33. Decibel Unit of Sound unit of sound
expressed in terms of the logarithm of their intensity
a 10 fold increase in energy is 1 bel
0.1 bel is a decibel
1 decibel is an increase in sound energy of 1.26 times
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34. Central Auditory Pathway
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35. some fibers pass to the ipsilateral superior olivary nucleus.
2nd order neurons project through trapezoid body to the contralateral superior olivary nucleus.
some fibers pass to the ipsilateral superior olivary nucleus.
from superior olivary nucleus to inferior colliculus via the lateral lemniscus
from medial geniculate to auditory cortex
from inferior colliculus to medial geniculate
fibers enter dorsal and ventral cochlear nuclei of the upper part of the medulla.
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36. Auditory Cortex and Association Areas
- arranged by tonotopic maps
- high frequency sounds at
one end of map
- low frequency sounds at
other end
- discrimination of sound
patterns
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37. Determining the Direction of Sound
superior olivary nucleus divided into lateral and medial nuclei
lateral nuclei detects direction by the difference in sound intensities between the two ears
medial nuclei detects direction by the time lag between acoustic signals entering the ears
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38. Deafness nerve deafness
impairment of the cochlea or the auditory nerve
conduction deafness
impairment of tympanic membrane or ossicles
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39. Audiometery
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40. Weber test
Rinne test
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41. The Internal Ear (Site of Equilibrium Receptors)
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42. Thank You
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